In one study, the University of Oxford documented what happens when players engage in games of prisoner's dilemma. During the game, players can either compete with their opponent to try to win everything; or, they can cooperate and divide the reward.

It turned out that tryptophan-deprived players were significantly less likely to cooperate. "I was surprised by the magnitude of the effect," says Robert Rogers, of Oxford's Department of Psychiatry.

Competing Amino Acids

When people take tryptophan supplements, the purified compound can get to the brain with relative ease.

But after a turkey feast, the tryptophan molecules are slowed down. They're, in essence, delivered to the body as part of a package with lots of competing amino acids.

"If you feed people a high protein diet, you don't get changes in brain tryptophan or serotonin," explains Stipanuk.

The reason? A traffic jam in crossing the blood-brain barrier. After a meal, all of the amino acids contained in protein-rich food are competing to get from the bloodstream to the brain. Think of it as a revolving door.

"If you're a tryptophan molecule, you want to get through that door," says Simon Young of McGill University in Montreal. But with lots of other amino acids competing, it takes you longer to get in.

Having heard the evidence, Robyn MacAdams, traveling through Union Station, has a different theory about one possible cause of the post-Thanksgiving feast letdown.

"Maybe it's because I'm around my family, so I just want to sleep," MacAdams says jokingly.

And social interactions around the holiday may trump specific chemical changes brought on by food.

"Here we are thinking that some substance is going to alter our destinies," says Christine DuBois of Johns Hopkins University. Instead, we need to realize that the way we treat people and interact with people is so paramount."